1. Field of the Invention
This invention relates in general to a method for treating a surface of a silicon single crystal and a method for manufacturing a silicon single crystal thin film, and more particularly to a method for removing the natural oxide film formed on the surface of a silicon single crystal before growing a silicon single crystal thin film and such on the silicon single crystal.
2. The prior Art
A silicon dioxide film with a thickness of 10-15 angstroms is generally formed as a natural oxide filem on the surface of a silicon single crystal substrate used for manufacturing semiconductor devices. When growing a silicon single crystal thin film and such on this silicon single crystal substrate, it is indispensable to remove the natural oxide film beforehand. If the natural oxide film is left on the surface, then the information about the crystal axis of the silicon single crystal substrate is hidden. Therefore, when a silicon single crystal thin film is grown on the silicon single crystal substrate by means of, for example, vapor phase epitaxial growth without removing the natural oxide film, then the crystallinity of the silicon single crystal thin film cannot be maintained.
Conventionally, methods used to remove the natural oxide film formed on the surface of a silicon single crystal substrate include a method in which a silicon single crystal substrate is heated up to about 1100.degree. C. and held in hydrogen gas to reduce the natural oxide film and a method in which it is also held in a high temperature environment and hydrogen chloride gas is fed to remove the natural oxide film along with silicon on the substrate surface. Also proposed is a method in which a treatment is conducted similarly at a high temperature using hydrogen fluoride gas which has the same effect on silicon as does hydrogen chloride.
On the other hand, a method which has been proposed for the removal of the natural oxide film without heating the silicon single crystal substrate comprises use of a three-ingredient mixed gas of hydrogen fluoride gas, nitrogen gas and water vapor or a three-ingredient mixed gas of hydrogen fluoride gas, nitrogen gas and alcohol vapor to convert the natural oxide film into silicon tetra fluoride for removal.
However, of these methods, the methods in which the treatment is conducted at a high temperature above 1100.degree. C. may cause out-diffusion of the dopant such as boron to vaporize it from the silicon single crystal substrate, thus disrupting the dopant distribution in the cross-sectional direction of the silicon single crystal substrate. In this case, a layer with a reduced dopant concentration is formed from the surface to a depth of approximately 1 micrometer, and also a so-called auto-doping phenomenon occurs, frequently resulting in a dopant concentration in the direction of the diameter of the silicon single crystal thin film which is different from the intended value.
Further, the method described above etches the silicon single crystal along with the natural oxide film and, as a result, the surface of the silicon single crystal substrate becomes uneven after the removal of the natural oxide film, and the surface condition may not be appropriate for conducting the gas phase growth of a silicon single crystal thin film and such.
On the other hand, for the methods which use a mixed gas of hydrogen fluoride gas, nitrogen gas and water vapor or a mixed gas of hydrogen fluoride gas, nitrogen gas and alcohol vapor, hydrogen fluoride gas severely etches the silicon surface and unevenness may be formed on the surface of the silicon single crystal substrate in the same manner as described above. The primary reason for this is because water vapor or alcohol vapor which is added to accelerate the removal of the natural oxide film significantly increases the corrosiveness of hydrogen fluoride gas.
Therefore, a method which uses a mixed gas with hydrogen fluoride anhydride is also investigated. In this case, although the removal of the natural oxide film does proceed, there are problems in reproducibility and the stability of the removal rate and the yield is low, making it unsuitable as an industrial process.